Conventionally, fiber-reinforced composite materials (i.e., fiber-reinforced resin materials), which are obtained by impregnating reinforcing fibers with a matrix resin such as a thermosetting resin or a thermoplastic resin, and are lighter and more elastic than metal materials, and are also stronger than when only a resin material is used, are drawing attention for application to members of vehicles and the like.
In order to utilize the inherent characteristics of reinforcing fibers, i.e., high strength and high elasticity against tension, using a sheet, which is obtained by using reinforcing fibers such as continuous fibers as a reinforcing-fiber base material, and impregnating or semi-impregnating the reinforcing-fiber base material with a matrix resin in advance, as a prepreg sheet (i.e., a fiber-reinforced resin sheet) is common.
Meanwhile, a fiber-reinforced resin sheet that is impregnated with a thermoplastic resin as a matrix resin is drawing attention as a material with added values because it is superior to a sheet impregnated with a thermosetting resin in terms of toughness, short-time moldability, recyclability, and the like due to the characteristics of the thermoplastic resin.
However, thermoplastic resins typically have higher viscosity than thermosetting resins, and in order to mold a thermoplastic resin, the resin should be heated to a high temperature. Thus, it would be not easy even to mold a thermoplastic resin into a film with uniform thickness. As such a technique, for example, there is known a technique of molding a thermoplastic resin into a film form by introducing a sheet-form molten resin, which has been extruded using an extruder, into the gap between a main roll made of metal and a press roll made of metal, and causing the thin outer cylinder of the press roll to deform along the shape of the circumferential surface of the main roll, and pressing the resin between the rolls while cooling the resin (for example, see Patent Literature 1).
As described above, it would be not easy even to mold a thermoplastic resin into a film form. Further, manufacturing a fiber-reinforced resin sheet containing a thermoplastic resin as a matrix resin is a more difficult technique than using a thermosetting resin. Thus, various researches have been conducted for such techniques.
For example, as a method for manufacturing a fiber-reinforced resin sheet by impregnating a reinforcing-fiber base material with a thermoplastic resin, there have been proposed a method of pressing a softened thermoplastic resin film against a reinforcing-fiber base material, using a pair of metal press rolls, thereby impregnating the reinforcing-fiber base material with the thermoplastic resin (for example, see Patent Literature 2), and a method of cutting reinforcing fibers into given lengths, mixing the reinforcing fibers with resin powder, and melting the thermoplastic resin in the gaps between a plurality of pairs of thermal pressure rolls, thereby impregnating spaces between the reinforcing fibers with the molten thermoplastic resin (for example, see Patent Literature 3).